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SP-0426 DOSIMETRIC COMMISSIONING OF LINACS IN VMAT MODE
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the additional uncertainty caused by the need for adopting new Ivalues and stopping powers, it can be concluded that a recommendation for changes to the data in TRS-398 needs to wait. In addition, the need for finding a solution which provides consistent dose determination in the entire range of radiotherapy beams, irrespective of the procedure used for deriving the necessary data, remains a major priority. A joint effort between IAEA, AAPM and other organizations is being made to issue international recommendations for the dosimetry of small beams photon fields used in stereotactic and intensity modulated radiotherapy. These recommendations are based on the formalism by Alfonso et al (Med Phys 2008) which establishes a link to current dosimetry protocols with the addition of a new type of kQ factors. These are being compiled for a variety of radiation generators and detectors. SP-0426 DOSIMETRIC COMMISSIONING OF LINACS IN VMAT MODE K. Rosser1 1 The Royal Marsden NHS Foundation Trust, Medical Physics, London, United Kingdom Purpose: The increased use of VMAT in routine clinical practice has raised the need for standardizing the dosimetric commissioning of Linacs in this treatment mode. This paper will describe the basic operation of Linacs in VMAT mode and the dosimetric challenges that they pose. It will then discuss the standard dosimetry and finally review routine quality assurance methods to maintain accurate treatment delivery. Methods: VMAT uses one or more gantry arcs to deliver dose from a range of coplanar or non-coplanar directions. This is achieved by simultaneously varying the gantry speed, multileaf collimator (MLC) leaf speed and dose rate in order to create the optimum treatment plan in the lowest possible treatment time. The calibration of Linacs in VMAT mode is a challenge to classical dosimetry protocols that involve static fields of moderately large size. With VMAT the beams are dynamic in orientation and aperture shape, and may include small apertures. The scatter conditions are expected to differ in such beams, compared to classical open fields, and therefore calibrations carried out under broad-beam conditions may not be relevant. In this paper a formalism published by the International Atomic Energy Agency working party to improve the dosimetry for small and non-standard fields [1] will be applied to VMAT beams [2]. Appropriate quality assurance is required for rotational therapy as it incorporates capabilities such as variable dose-rate, variable gantry speed, and accurate and fast dynamic multileaf collimators. This paper will discuss routine QA techniques used in the clinic [3], [4]. Including a review of commercial devices to verify patient specific treatment plans. Conclusion: VMAT is becoming routine in the clinic. It is essential that the dosimetry is set up accurately at commissioning and a formal QA program is established throughout the life of the machine. External audits [5] provide a tool to check the dosimetric accuracy established at commissioning. References [1] Alfonso R, Andreo P, Capote R, Saiful Huq M, Kilby W, Kjäll P, Mackie T R, Palmans H, Rosser K, Seuntjens J, Ullrich W and Vatnitsky S, ‘ A new formalism for reference dosimetry of small and nonstandard fields’ Med. Phys. 35, 5179-5186 , 2008. [2] Rosser K E and Bedford J L, ‘Application of a new dosimetry formalism to vulmetric modulated arc therapy (VMAT)’, Phys. Med. Biol., 54, 7045-7061, 2009. [3] Bedford J L and Warrington A P, ’Commissioning of volumetric modulated arc therapy (VMAT)’, Int. J. Radiation Oncology Biol. Phys., 73, 537–545, 2009. [4] Ling C C, Zhang P, Archambault Y, Bocanek J, Tang G, Phil M and Losasso, ‘Commissioning and quality assurance of RapidArc radiotherapy delivery system’, Int. J. Radiation Oncology Biol. Phys., 72, 575–581, 2008. [5] Hussein M, Tsang Y, Thomas R, Gouldstone C, Maughan D, Snaith J, Bolton S, Clark C H. ‘Rotational radiotherapy in the UK - A pilot audit’, abstract submitted to ESTRO 31, 2012.
ESTRO 31
SP-0427 DOSIMETRY IN STRONG MAGNETIC FIELDS; ISSUES AND OPPORTUNITIES B. Raaymakers1, K. Smit1, G.H. Bol1, M.R. Van den Bosch1, S.P.M. Crijns1, J.G.M. Kok1, J.J.W. Lagendijk1 1 U.M.C. Utrecht, Academic Physics, Utrecht, The Netherlands There are three hybrid MRI radiotherapy devices under construction. A bi-planar MRI combined with a 6 MV accelerator by the group of in Edmonton, Canada, a 0.35 T MRI with 60Co radiotherapy device by the Viewray company, U.S.A. and our 1.5 T MRI with a 6 MV accelerator at the UMC Utrecht, The Netherlands in collaboration with Elekta, U.K. and Philips, The Netherlands. The goal of these systems is to use the excellent soft-tissue contrast of MRI for real-time radiotherapy treatment guidance. With these systems, radiation delivery will take place in the presence of a magnetic field. Although the photon beams are not affected by the magnetic field, the secondary electrons are experiencing the Lorentz force and as such the dose distribution will be affected by the magnetic field, especially at tissue-air interfaces. This presentation will detail the impact of the magnetic on the dose distribution, both for single beams and for IMRT dose distributions. It will be shown that these effects vary but occur for all magnetic field strenghts, and can be relatively large. Fortunately they are also fully deterministic and can be accounted for by including the impact of the magnetic field in the treatment planning. Moreover, radiation dosimeter are affected by the magnetic field. The impact of the magnetic field for both absolute dosimetry by means of a Farmer ionisation chamber as well as for relative dosimetry by means of pinpoint air-filled ionisations chambers, liquid filled ionisation chamber and the edge solid state detector will be discussed. Dosimetry in magnetic fields has to account for the impact of the magnetic field on the dose distribution, and has to account for a change in response of radiation detectors due to the presence of the magnetic field. SP-0428 DOSIMETRIC CONCEPTS BASED ON PARTICLE TRACK STRUCTURE H. Rabus1 1 Physikalisch-Technische Bundesanstalt (PTB), Fundamentals of Dosimetry, Braunschweig, Germany Purpose/Objective: Currently, the established way to characterize radiation exposure is based on quantification in terms of absorbed dose to water complemented by a set of different quantities to account for radiation quality. These quantities range from the phenomenological kQ correction factors in external beam therapy to more elaborate concepts, such as the radiation weighting factors in nuclear medicine and the relative biological effectiveness in ion beam therapy. While radiation quality is generally specified by the way the radiation is produced (e.g. acceleration voltage and filtering or particle type and energy), a redefinition of this dosimetric concept based on measurable properties of the microscopic particle track structure may open the way to account for radiation quality in a uniform way. Material/method: In micro- and nanodosimetry, ionising radiation track structure is characterised by the probability distributions of stochastic quantities like lineal energy or ionisation cluster size produced in targets having the size of a cell nucleus or a short segment of DNA, respectively. Experimental techniques have been developed that determine these probability distributions in gas targets equivalent to the aforementioned microscopic targets in tissue by virtue of a density scaling relation. Track structure codes, i.e. numerical tools simulating the radiation transport step-by-step by taking each individual interaction into account, are then used to establish the link to the microscopic details of track structure in tissue. Results: For ionisation cluster size distributions, the validity of the scaling relation was shown experimentally to hold not only for simple gases like nitrogen and propane, but also for more complex molecules like tetrahydrofuran, a homologue of the deoxyribose in the DNA backbone. Simulations based on an upgrade of the PTB track structure code that includes recent experimentally determined electron scattering cross section data of tetrahydrofuran showed a 40% reduction in mean ionisation cluster size when a DNA target is modelled as consisting of tetrahydrofuran instead of water. Parameters derived from ionization cluster size distributions of ion beams showed similar dependence on radiation quality as the probability of induction of biological end points related to early DNA damage. Conclusion: While the applicability of the scaling relation was
the additional uncertainty caused by the need for adopting new Ivalues and stopping powers, it can be concluded that a recommendation for changes to the data in TRS-398 needs to wait. In addition, the need for finding a solution which provides consistent dose determination in the entire range of radiotherapy beams, irrespective of the procedure used for deriving the necessary data, remains a major priority. A joint effort between IAEA, AAPM and other organizations is being made to issue international recommendations for the dosimetry of small beams photon fields used in stereotactic and intensity modulated radiotherapy. These recommendations are based on the formalism by Alfonso et al (Med Phys 2008) which establishes a link to current dosimetry protocols with the addition of a new type of kQ factors. These are being compiled for a variety of radiation generators and detectors. SP-0426 DOSIMETRIC COMMISSIONING OF LINACS IN VMAT MODE K. Rosser1 1 The Royal Marsden NHS Foundation Trust, Medical Physics, London, United Kingdom Purpose: The increased use of VMAT in routine clinical practice has raised the need for standardizing the dosimetric commissioning of Linacs in this treatment mode. This paper will describe the basic operation of Linacs in VMAT mode and the dosimetric challenges that they pose. It will then discuss the standard dosimetry and finally review routine quality assurance methods to maintain accurate treatment delivery. Methods: VMAT uses one or more gantry arcs to deliver dose from a range of coplanar or non-coplanar directions. This is achieved by simultaneously varying the gantry speed, multileaf collimator (MLC) leaf speed and dose rate in order to create the optimum treatment plan in the lowest possible treatment time. The calibration of Linacs in VMAT mode is a challenge to classical dosimetry protocols that involve static fields of moderately large size. With VMAT the beams are dynamic in orientation and aperture shape, and may include small apertures. The scatter conditions are expected to differ in such beams, compared to classical open fields, and therefore calibrations carried out under broad-beam conditions may not be relevant. In this paper a formalism published by the International Atomic Energy Agency working party to improve the dosimetry for small and non-standard fields [1] will be applied to VMAT beams [2]. Appropriate quality assurance is required for rotational therapy as it incorporates capabilities such as variable dose-rate, variable gantry speed, and accurate and fast dynamic multileaf collimators. This paper will discuss routine QA techniques used in the clinic [3], [4]. Including a review of commercial devices to verify patient specific treatment plans. Conclusion: VMAT is becoming routine in the clinic. It is essential that the dosimetry is set up accurately at commissioning and a formal QA program is established throughout the life of the machine. External audits [5] provide a tool to check the dosimetric accuracy established at commissioning. References [1] Alfonso R, Andreo P, Capote R, Saiful Huq M, Kilby W, Kjäll P, Mackie T R, Palmans H, Rosser K, Seuntjens J, Ullrich W and Vatnitsky S, ‘ A new formalism for reference dosimetry of small and nonstandard fields’ Med. Phys. 35, 5179-5186 , 2008. [2] Rosser K E and Bedford J L, ‘Application of a new dosimetry formalism to vulmetric modulated arc therapy (VMAT)’, Phys. Med. Biol., 54, 7045-7061, 2009. [3] Bedford J L and Warrington A P, ’Commissioning of volumetric modulated arc therapy (VMAT)’, Int. J. Radiation Oncology Biol. Phys., 73, 537–545, 2009. [4] Ling C C, Zhang P, Archambault Y, Bocanek J, Tang G, Phil M and Losasso, ‘Commissioning and quality assurance of RapidArc radiotherapy delivery system’, Int. J. Radiation Oncology Biol. Phys., 72, 575–581, 2008. [5] Hussein M, Tsang Y, Thomas R, Gouldstone C, Maughan D, Snaith J, Bolton S, Clark C H. ‘Rotational radiotherapy in the UK - A pilot audit’, abstract submitted to ESTRO 31, 2012.
ESTRO 31
SP-0427 DOSIMETRY IN STRONG MAGNETIC FIELDS; ISSUES AND OPPORTUNITIES B. Raaymakers1, K. Smit1, G.H. Bol1, M.R. Van den Bosch1, S.P.M. Crijns1, J.G.M. Kok1, J.J.W. Lagendijk1 1 U.M.C. Utrecht, Academic Physics, Utrecht, The Netherlands There are three hybrid MRI radiotherapy devices under construction. A bi-planar MRI combined with a 6 MV accelerator by the group of in Edmonton, Canada, a 0.35 T MRI with 60Co radiotherapy device by the Viewray company, U.S.A. and our 1.5 T MRI with a 6 MV accelerator at the UMC Utrecht, The Netherlands in collaboration with Elekta, U.K. and Philips, The Netherlands. The goal of these systems is to use the excellent soft-tissue contrast of MRI for real-time radiotherapy treatment guidance. With these systems, radiation delivery will take place in the presence of a magnetic field. Although the photon beams are not affected by the magnetic field, the secondary electrons are experiencing the Lorentz force and as such the dose distribution will be affected by the magnetic field, especially at tissue-air interfaces. This presentation will detail the impact of the magnetic on the dose distribution, both for single beams and for IMRT dose distributions. It will be shown that these effects vary but occur for all magnetic field strenghts, and can be relatively large. Fortunately they are also fully deterministic and can be accounted for by including the impact of the magnetic field in the treatment planning. Moreover, radiation dosimeter are affected by the magnetic field. The impact of the magnetic field for both absolute dosimetry by means of a Farmer ionisation chamber as well as for relative dosimetry by means of pinpoint air-filled ionisations chambers, liquid filled ionisation chamber and the edge solid state detector will be discussed. Dosimetry in magnetic fields has to account for the impact of the magnetic field on the dose distribution, and has to account for a change in response of radiation detectors due to the presence of the magnetic field. SP-0428 DOSIMETRIC CONCEPTS BASED ON PARTICLE TRACK STRUCTURE H. Rabus1 1 Physikalisch-Technische Bundesanstalt (PTB), Fundamentals of Dosimetry, Braunschweig, Germany Purpose/Objective: Currently, the established way to characterize radiation exposure is based on quantification in terms of absorbed dose to water complemented by a set of different quantities to account for radiation quality. These quantities range from the phenomenological kQ correction factors in external beam therapy to more elaborate concepts, such as the radiation weighting factors in nuclear medicine and the relative biological effectiveness in ion beam therapy. While radiation quality is generally specified by the way the radiation is produced (e.g. acceleration voltage and filtering or particle type and energy), a redefinition of this dosimetric concept based on measurable properties of the microscopic particle track structure may open the way to account for radiation quality in a uniform way. Material/method: In micro- and nanodosimetry, ionising radiation track structure is characterised by the probability distributions of stochastic quantities like lineal energy or ionisation cluster size produced in targets having the size of a cell nucleus or a short segment of DNA, respectively. Experimental techniques have been developed that determine these probability distributions in gas targets equivalent to the aforementioned microscopic targets in tissue by virtue of a density scaling relation. Track structure codes, i.e. numerical tools simulating the radiation transport step-by-step by taking each individual interaction into account, are then used to establish the link to the microscopic details of track structure in tissue. Results: For ionisation cluster size distributions, the validity of the scaling relation was shown experimentally to hold not only for simple gases like nitrogen and propane, but also for more complex molecules like tetrahydrofuran, a homologue of the deoxyribose in the DNA backbone. Simulations based on an upgrade of the PTB track structure code that includes recent experimentally determined electron scattering cross section data of tetrahydrofuran showed a 40% reduction in mean ionisation cluster size when a DNA target is modelled as consisting of tetrahydrofuran instead of water. Parameters derived from ionization cluster size distributions of ion beams showed similar dependence on radiation quality as the probability of induction of biological end points related to early DNA damage. Conclusion: While the applicability of the scaling relation was